Frequency control network

ABSTRACT

A FREQUENCY SHIFT OSCILLATOR INCLUDES A FIRST FREQUENCY DETERMINING CIRCUIT WHEREBY THE OSCILLATOR NORMALLY FUNCTIONS AT A FIRST FREQUENCY. A SECOND CIRCUIT HAVING INPUT AND OUTPUT TERMINALS IS ARRANGED TO BE SWITCHED BETWEEN PREDETERMINED POINTS IN THE FIRST CIRCUITS, SHIFTING THE FREQUENCY OF THE OSCILLATOR TO A SECOND FREQUENCY. THE PREDETERMINED POINTS IN THE FIRST CIRCUIT AND THE INPUT-OUTPUT TERMINALS OF THE SECOND CIRCUIT ARE ALL MAINTAINED AT SUB-   STANTIALLY THE SAME CONSTANT POTENTIAL IRRESPECTIVE OF WHETHER OR NOT THE SECOND CIRCUIT IS SWITCHED BETWEEN THE PREDETERMINED POINTS IN THE FIRST CIRCUIT, AVOIDING THE GENERATION OF TRANSIENTS WHICH WOULD OTHERWISE OCCUR UPON THE SWITCHING OF THE SECOND CIRCUIT IN AND OUT OF THE FIRST CIRCUIT.

F8). 19. w F. 5T RC. FREQUENCY SHIFT OSCILLATOR WHICH AVOTDS THEGENERATION OF TRANSlENTS Original Filed May 23, 1969 FREQUENCY CONTROLNETWORK SECOND FREQUENCY H 1 DETERMINING PORTION j I I I I I I I I I I II I I FIRST FREQUENCY DETERMINING I25 --46UTPUT PORTION R E 0 I 2 O L IQ M d u 4 m I I I I I I I I I I l l l I||I 6 00 m I I vI I mm 9 U S KM mu u AIII mm m TWS WLLA NAN TBE EMU OMM OIM DIS PA E M ATTORNEY UnitedStates Patent Office Reissued Feb. 19, 1974 27,924 FREQUENCY SHIFTOSCILLATOR WHICH AVOIDS THE GENERATION F TRANSIEN'IS William FrederickHingston, Needham, Mass, assignor to RCA Corporation Original No.3,618,132, dated Nov. 2, 1971, Ser. No. 827,188, May 23, 1969.Application for reissue Aug. 28, 1972, Ser. No. 284,091

Int. Cl. H0311 5/20,- 1103c 3/06 US. Cl. 331179 7 Claims Matter enclosedin heavy brackets appears in the original patent but forms no part ofthis reissue specification; matter printed in italics indicates theadditions made by reissue.

ABSTRACT OF THE DISCLOSURE A frequency shift oscillator includes a firstfrequency determining circuit whereby the oscillator normally functionsat a first frequency. A second circuit having input and output terminalsis arranged to be switched between predetermined points in the firstcircuit, shifting the frequency of the oscillator to a second frequency.The predetermined points in the first circuit and the input outputterminals of the second circuit are all maintained at substantially thesame constant potential irrespective of whether or not the secondcircuit is switched between the predetermined points in the firstcircuit, avoiding the generation of transients which would otherwiseoccur upon the switching of the second circuit in and out of the firstcircuit.

This invention relates to an oscillator, and more particularly, to afrequency shift oscillator.

Frequency-shifted oscillators are currently experiencing widespreadapplication in the communication of digital information. One of theseapplications is concerned with communicating digital information incertain systems which require conversion of the digital information tothe audiofrequency range. As a result, numerous techniques have beendeveloped for achieving this end. Generally, these techniques areconcerned with converting two-level binary data into audio tonescorresponding to the respective binary levels. Some of these techniquesinclude switching between multiple oscillators; switched, phase shift,RC oscillators; voltage-controlled multivibrator oscillators; andvariable reactance, phase shift RC oscillators among others. However,the previously known techniques all represent various compromises withdata pulse distortion, frequency stability, frequency shift range,circuit complexity, and cost.

Ideally, jitterless data modulation over a wide data speed range, withnarrow or wide frequency shifts can be accomplished by an RC phase shiftoscillator. However, the problem here is that introduction ofvariable-frequency-determining elements to change the frequency may alsointroduce DC transient components into the RC network. Past elforts toresolve this problem in RC networks have met with little success. Whenan impedance is introduced in an RC network to shift frequencies, twothings occur. First, due to its initial condition (having a potentialdifferent from that across the RC network), the impedance presents atransient. Second, since the added impedance shunts the original RCnetwork to shift the frequency, the DC potential difference in thecombined network would ordinarily shift, and further transients would beintroduced due to these shifts in potential. Transients so produceddisrupt the wave train generated by the oscillator producing distortionwhich nullifies the effectiveness of such circuits.

It is an object of the present invention to provide an improvedfrequency shift oscillator that is substantially free of suchtransients.

An oscillator is provided herein that includes a frequency-determiningcircuit such that the oscillator normally functions at a firstfrequency. Means are provided for selectively changing the parameters ofthe circuit in a manner to switch the oscillator between the first and asecond frequency. The potentials in the circuit which, if permitted todiffer with respect to one another, would produce transients upon theoscillator being switched between the first and second frequency, areall maintained substantially constant and equal, avoiding theproductions of such transients.

In the drawings:

FIG. 1 is a conceptual block diagram of an embodiment of a frequencyshift oscillator according to the invention.

FIG. 2 is a circuit diagram illustrating one form which the embodimentof FIG. 1 may take.

In FIG. 1, there is shown an amplifier 120 coupled in series with afirst frequency-determining portion of frequency-controlling network115. The amplifier 120 is selected to have a high output impedance and alow input impedance. The gain of the amplifier 120 is selected inconjunction with the frequency-determining network to provide unity loopgain. Portion or circuit 100 may be an RC frequency-determininggrouping. In parallel with the resistance segment of first portion 100is a second frequency-determining portion or circuit 102 which may be aresistive grouping in series with a two-pole switch 104 having open andclosed switch conditions. Portions 100 and 102 have predetermined pointslocated therein such as points (a, b) and (c, d), respectively. Thesepoints define the boundaries of the respective resistive segments inportions 100 and 102. Potential-establishing means 106 is shown coupledto the entire circuit as are potential-maintaining means 108 and a DCsupply 110.

In the operation of the oscillator shown conceptually in FIG. 1, firstportion 100 has an RC grouping that is preselected to generate the firstor lower of the two preselected frequencies. When switch 104 is closed,the resistive grouping of second portion 102 is coupled in parallel withthe resistive grouping segment of portion 100, presenting a lowerresistive frequency-determining value in the network 115, therebyshifting the oscillator to a second or higher frequency. The resistivevalues between points (a) and (b) in portion 100, and (c) and (d) inportion 102 are selected in accordance with the predeterminedfrequencies. The output is taken from terminal 125.

Potential-establishing means 106 establishes the DC component of the ACsignal at points (a b, c, and d) to be substantially equal.Potential-maintaining means 106 maintains the DC component of the ACsignal at these points substantially constant. Thus when switch 104 isclosed, points (a, b, c, and d) will be at the same relative potential,and there will be no initial potential drop across portion 102. However,once the resistive values across points (d) and (c) have been coupledacross points (a) and (b), the second resistive value in the networkbeing presented by the combined resistive values of the two portionswould ordinarily present a change in potential across the combinedportions. This is prevented by means 108. Therefore, there are nosignificant potential shifts across the frequency-determining portionsof network 115, thereby eliminating any transients which would otherwiseoccur.

Referring now to FIG. 2, there is shown a frequency shift oscillator ofthe Wien Bridge type including a noninverting amplifier 25 consisting oftransistors 40, 50, and 60 of the NPN type, such as are found in RCAintegrated circuit CA 3046. Base 61 of transistor 60 is connected to ajunction 26 as are resistor 46 and capacitor 95. Emitter 63 is connectedto resistor 48, which in turn is connected to junction 27 as areresistor 47 and terminal 69 which has a negative potential V (e.g., 7.5volts) applied thereto. Collector 62 of transistor 60 is connected toresistor 49 and base 51 of transistor 50. Emitter 53 of transistor 50 isconnected to base 41 of transistor 40 and emitter 43 of transistor 40 isconnected to the other sides of resistors 47 and 46. Collector 52 oftransistor 50 is connected to junction 28 as is the other side ofresistor 49 and junction 36. Resistors 46, 47, and 48 establish the gainof the amplifier. In addition, these resistors complete a feedback pathfrom junction 26 to emitters 43 and 63. Resistor 49 is the load resistorfor collector 62.

Coupled to amplifier 25 is frequency-controlling network 35 consistingof two frequency-determining portions A and B. Portion A has twobranches 35a and 35b. Branch 35a includes a resistive segment 98 inparallel with a reactive segment shown as a capacitor 94, one end of thebranch 35a being connected to junction (s) as is junction 36. The otherend of the branch 35a is connected to junction (t) as is collector 42 oftransistor 40, output terminal 68, junction (r) and branch 35b.Resistive segment 96 of branch 35b is connected between junctions (t)and (u), while reactive segment capacitor 95 is serially connected toresistive segment 96 by way of junction 26 and (u).

Potential-establishing means 55 consists of amplifier described aboveand DC voltage supply V (e.g.: +7.5 volts) connected to resistor 99which in turn is connected to junction 36. Connected to junction 36 isjunction (s) and one end of a resistor 97 and of a capacitor 70. Theother side of resistor 97 and of capacitor 70 is grounded at junction(q). Capacitor 70 is a low-impedance AC decoupling component and issubstantially greater than the value of reactive segment 94 e.g., 100times). Means 55, together with amplifier 25, establishes the ACpotential at predetermined points such as junctions (t and u)substantially the same, that is, at virtual DC ground.

PotentiaLmaintaining means 45 is a voltage divider consisting of DCvoltage supply V resistors 97, 98, and 99 connected in the manner shownabove. Resistor 99 is the series-dropping resistor, resistor 97 is thebleeding resistor and resistor 98 is the load resistor. By setting thevalue of resistors 98 substantially greater than the value of resistor97 (e.g.: approximately 10 times), the current flowing through resistor98 will be much less than the current flowing through resistor 97 and99. Thus any resistive fluctuations in the entire circuit will not causeequivalent DC potential fluctuations, which in this case would causedeleterious transients in the output waveform. The potential drop acrossjunctions (s, q) is made equal to the drop across junctions (s, t),wherein the DC potential at (t) is d a virtual ground as compared toactual ground at (q).

Frequency-determining portion B consists of resistive segments 86 and88, and switching transistors and 30, which are utilized for thetwo-pole switch 104 in FIG. 1,

and may be of the type 2N3704. Segment 86 is connected between junction(u) and the collector 32 of transistor 30. Resistive segment 88 isconnected to junction (r), which in turn is connected to emitter 33 oftransistor 30. The other side of resistive segment 88 is connected tothe collector 22 of transistor 20. Emitter 23 of transistor 20 isconnected to ground at junction (q). Bases 21 and 31 of transistors 20and 30, are respectively connected to resistors 82 and 84, which in turnare connected to junction 39 as is resistor 80. The other side ofresistor 80 is connected to terminal 67 at which a positive DC voltage V(e.g.: +7.5 volts) is applied. Segments 86 and 88 are substantiallyequal. The turn-on collector-emitter resistance of transistors 20 and 30is substantially lower than the resistive value of segments 86 and 88(e.g.: a few ohms). The base resistors 82 and 84 are substantially highvalues (e.g.: 150K ohms) in order to minimize the current flow throughthe switching transistors. Keying transistor 10 may be the type 2N3704and has its collector 12 connected to junction 39, its emitter 13grounded, and keying ing binary input signal applied to its base 11 frominput terminal 66.

The operation of the oscillator of FIG. 2 will now be described.Amplifier 25, resistive segments 96 and 98 and reactive segments 94 and95 function as a Wien-Bridgetype oscillator, having positive feedbackvia lead 45. The Irequencydetermining value of network 35 is establishedby the value of resistive segments 98, and 96 which may be approximately2,970 ohms each, and reactive segments 94 and 95 which may beapproximately 0.05 [LF each. That is, branch 35a and branch 35b havesubstanially equal resistive and reactive segments. Additionally, thegain of the amplifier 25 is predetermined to establish unit loop gain.In this case, the forward gain required is 3. In a balanced Wein-typebridge having two resistive-reactive branches, the followingrelationship holds:

F=1/21rRC where:

F=Frequency of the bridge R=Resistive value of one branch of the bridgeC=Capacitive value of the same branch A=Gain of the amplifier and wherethe resistive and capacitive values of the two branches aresubstantially equal. Therefore, the frequency is an inverse function ofthe resistive and reactive capacitive value in one branch (35a) of thefrequency-determining portion. In order to shift the frequency of theoscillator, it is necessary to either shift the value of the resistiveor reactive segments, or both, as the case may be. In this case, theresistive value is shifted. In order to maintain the balanced bridgecondition, the value of the resistive segments in both branches ofportion A of the network 35 must remain substantially equal, regardlessof the frequency being generated. In the embodiment illustrated in FIG.2, the resistive segments 86 and 88 are equal to each other. By couplingpredetermined points such as junction (r) to junction (t), resistivesegment 86 is effectively in parallel with segment 96 and segment 88 isin parallel with segment 98 with respect to AC voltage by way ofdecoupling capacitor 70. Segments 86 and 88 are coupled into thefrequency-determining network 35 when the transistors 20 and 3-0 areconducting. When they are nonconducting, the AC frequency-controlledvoltage is affected only by the frequency-determining value presented byfrequency-determining portion A. When the transistors 20, 30 areswitched on, or become conducting, the AC voltage is effected by thevalue presented by frequency-determining portions A and B combined. Inorder to prevent the generation of undesired transients, the DCcomponent of the AC signal at points across both frequency determiningbranches 35a and 35b must remain substantially constant and equal.Ordinarily, switching resistive elements in and out of a circuit as wellas changing the effective resistive values in the circuit generatetransients as indicated previously. By establishing the DC component ofthe AC signal substantially the same ground potential predeterminedpoints such as at (s, t, u, r, q) in the frequency-determining portionsof network 35 and by maintaining this DC component substantiallyconstant, this problem is avoided.

To achieve this end, voltage divider 45, which is illustrative only, andamplifier 25 establish the potential at junction (t) at virtual DCground. This is accomplished by negative DC voltage V (e.g.: 7.5 volts)applied at terminal 69, and by the values of resistors 46, 47, 48 inconjunction with the values of resistors 97 and 99 and resistive segment98. Note that not only is resistive segment 98 critical with respect tofrequency, but it is an important component with respect to establishingthe potential at junction (t), and also is the load on voltage dividerfor maintaining constant circuit voltage. This segment accomplishesthese functions without inhibiting the generation of the desiredpreselected first and second frequencies. Resistors 47 and 48 may havethe value of approximately 1,620 ohms, and 39 ohms, while resistors 97and 99 may have the value of 344 ohms and 680 ohms approximately. Loadresistor 49 may be 6,200 ohms. Re-

sistance 46 is determined empirically by placing an additionalresistance (not shown) in parallel with 3,640 ohms. As a result,junctions (q) and (t) are virtually at the same potential (DC ground).Therefore, all intervening serially connected points such as junctions(r) and (u) will be at the same potential. Capacitor 95 blocks the DCpotential at junction (u) and capacitor 70 bypasses to ground any ACsignal that appears at junction (s). Resistor 80, the collector load fortransistor 10, may be 3,900 ohms. By making the base resistors 82 and 84relatively high resistance, (e.g.: 150 K ohms), relatively low current(e.g.: 50 a.) flows through the collector-emitter circuit, presentingless than 1 percent distortion.

Normally, the applied voltage at terminal 67 (e.g.: +7.5 volts) biasestransistors 20 and 30 on. This couples frequency determining portion Bwith portion A, presenting a lower resistive value and generating asecond higher frequency. When a binary input signal is applied to base11 of transistor 10, the higher of the two binary levels turnstransistors on which then conducts. The potential at junction 39 goes toground and transistor and 30 are switched off. At this time, thefrequency-determining network consists only of branches 35a and 35b,presenting a higher resistive value and generating a first lowerfrequency. The output signal is taken from terminal 68, which signalconsists of a waveform rapidly alternating between two frequencieswithout transients.

What is claimed is:

[1. A frequency shift oscillator comprising,

a first resistive-capacitive frequency-determining circuit by which saidoscillator operates at a first frequency,

a second circuit which when coupled across a portion of said firstcircuit between predetermined points in said first circuit results insaid oscillator operating at a second frequency, said second circuithaving input and output terminals,

means for selectively coupling said second circuit at said terminalsbetween said points in said first circuit, and

means coupled to said first and second circuits for establishing andmaintaining substantially the same constant potential at said input andoutput terminals and said points irrespective of whether or not saidsecond circuit is coupled between said points in said first circuit,thereby avoiding transients in the output of said oscillator which wouldotherwise occur if the potential at any of said points and terminalsdiffered when the frequency of said oscillator is randomly shifted] [2.A frequency shift oscillator as claimed in claim 1 and wherein saidfirst circuit includes a resistive and capacitive element connected inseries,

said second circuit including a resistive element connected between saidterminals to cause said oscillator to operate at said second frequencyhigher than said first frequency when said second circuit is selectivelycoupled to said first circuit] [3. A frequency shift oscillator asclaimed in claim 1 and wherein said coupling means includes a switchingmeans connected in series with said second circuit between said pointsin said first circuit, and

means for placing said switching means in one state to couple saidsecond circuit between said points in said first circuit and in a secondstate to remove said second circuit from between said points] [4. Afrequency shift oscillator comprising,

a first frequency-determining circuit by which said oscillator operatesat a first frequency, said first circuit including a first resistiveelement in series with a second resistive element,

a second circuit including a switching means in series with aresistance, said second circuit when coupled across said first andsecond resistive elements causing said oscillator to operate at a secondfrequency, said second circuit having input and output terminals,

means for placing said switching means in one state to couple saidsecond circuit across said first and second resistive element and in asecond state to remove said second circuit from across said first andsecond resistive elements, and

means coupled to said circuits for establishing and maintainingsubstantially the same constant potential at said terminals of saidsecond circuit and at the ends of the series circuit formed by saidfirst and second resistive elements irrespective of whether or not saidsecond circuit is coupled across said series circuit] [5. A frequencyshift oscillator as claimed in claim 4 and wherein said second circuitincludes a first switching device in series with a third resistiveelement, the series arrangement of said first device and said thirdresistive element being connected across said first resistive element,and further including a second switching device in series with a fourthresistive element, the series arrangement of said second device and saidfourth resistive element being connected across said second resistiveelement,

said placing means serving to selectively operate said switching devicesto complete the connection of said third resistive element through saidfirst device across said first resistive element and the connection ofsaid fourth resistive element through said second device across saidsecond resistive element] [6. A frequency shift oscillator comprising:

a first frequency-detern1ining circuit including a plurality ofresistive and reactive frequency-determining elements interconnected forcausing said oscillator to operate at a first frequency and furtherincluding at least two points at predetermined locations therein, atleast one of said elements being connected between said two points,

a second circuit having input and output terminals each respective oneof which is coupled to a separate one of said two points and includingat least one resistive element connected between said terminals forshifting the frequency-determining value presented by said one elementin said first circuit for causing said oscillator to operate at a secondfrequency,

means coupled to said circuits for establishing and maintainingsubstantially the same constant potential at said two points and at saidinput and output terminals, which potential, if permitted to fluctuateor differ as between any of said points and said terminals, would causetransients to be generated when the frequency of said oscillator israndomly shifted, and

switching means having two states for selectively coupling said secondcircuit to said first circuit at said points and terminals in only oneof said states for controlling the generation by said oscillator ofeither one of said first and second frequencies] [7. A frequency shiftoscillator, comprising:

a first circuit including a plurality of frequency-determining elementsfor generating a first frequency and having a plurality of points atpredetermined locations therein, at least one of said elements beingconnected between one pair of said points, and another of said elementsbeing connected between a second different pair of said points,

a second circuit having a like number of points at predeterminedlocations therein and including at least one frequency-determiningelement connected between one pair of said second circuit points andanother frequency-determining element connected between a seconddifferent pair of said second circuit points, said onefrequency-determining element in said second circuit when coupled acrosssaid one element in said first circuit at said one pair of points andsaid other frequency-determining element in said second circuit whencoupled across said other element in said first circuit at said secondpair of points serving to shift the value of said elements in said firstcircuit causing said oscillator to operate at a second frequency;

means coupled to said first and second circuits for establishing andmaintaining substantially the same constant potential at all of saidpoints, which potential if permitted to fluctuate or ditfer between anyof said points when the value of said one element is shifted, wouldcause transients to be generated, and

switching means for selectively coupling said second circuit to saidfirst circuit at said points for controlling the generation by saidoscillator at either one of said first and second frequencies] 8. Afrequency shift oscillator including an amplifier comprising.

a first resistive-capacitive frequency-a etermining circuit D.C. coupledto said amplifier by which said oscillator operates at a firstfrequency,

a second circuit which when coupled across a portion of said firstcircuit between predetermined points in said first circuit results insaid oscillator operating at a second frequency, said second circuithaving input and output terminals,

means for selectively coupling said second circuit at said terminalsbetween said points in said first circuit, and

means coupled to said first and second circuits for establishing andmaintaining substantially the some constant potential at said input andoutput terminals and said points irrespective of whether or not saidsecond circuit is coupled between said points in said first circuit,thereby avoiding transients in the output of said oscillator which wouldotherwise occur if the potential at any of said points and terminalsdiflered when the frequency of said oscillator is randomly shifted.

9. A frequency shift oscillator including an amplifier comprising,

a first frequency-determining circuit D.C. coupled to said amplifier bywhich said oscillator operates at a first frequency, said first circuitincluding a first resistive element in series with a second resistiveelement,

a second circuit including a switching means in series with aresistance, said second circuit when coupled across said first andsecond resistive elements causing said oscillator to operate at a secondfrequency, said second circuit having input and output terminals,

means for placing said switching means in one state to couple saidsecond circuit across said first and second resistive elements and in asecond state to remove said second circuit from across said first andsecond resistive elements, and

means coupled to said circuits for establishing and maintainingsubstantially the some constant potential at said terminals of saidsecond circuit and at the ends of the series circuit formed by saidfirst and second resistive elements irrespective of whether or not saidsecond circuit is coupled across said series circuit.

10. A frequency shift oscillator including an amplifier comprising:

a first frequency-determining circuit D.C. coupled to said amplifierincluding a plurality of resistive and reactive frequency-determiningelements interconnected for causing said oscillator to operate at afirst frequency and further including at least two points atpredetermined locations therein, at least one of said elements beingconnected between said two points,

a second circuit having input and output terminals each respective oneof which is coupled to a separate one of said two points and includingat least one resistive element connected between said terminals forshifting the frequency-determining value presented by said one elementin said first circuit for causing said oscillator to operate at a secondfrequency,

means coupled to said circuits for establishing and maintainingsubstantially the some constant potential at said two points and at saidinput and output terminals which potential, if permitted to fluctuate ordifier as between any of said points and said terminals, would causetransients to be generated when the frequency of said oscillator israndomly shifted and switching means having two states for selectivelycoupling said second circuit to said first circuit at said points andterminals in only one of said states for controlling the generation bysaid oscillator of either one of said first and second frequencies.

1 I A frequency shift oscillator including an amplifier,

comprising:

a first circuit D.C. coupled to said amplifier including a plurality offrequency-determining elements for generating a first frequency andhaving a plurality of points at predetermined locations therein, atleast one of said elements being connected between one pair of saidpoints, and another of said elements being connected between a seconddifierent pair of said points,

a second circuit having a like number of points at predeterminedlocations therein and including at least one frequency-determiningelement connected between one pair of said second circuit points andanother frequency-determining element connected between a seconddifierent pair of said second circuit points, said onefrequency-determining element in said second circuit when coupled acrosssaid one element in said first circuit at said one pair of points andsaid other frequency-determining element in said second circuit whencoupled across said other element in said first circuit at said secondpair of points serving to shift the value of said elements in said firstcircuit causing said oscillator to operate at a second frequency,

means coupled to said first and second circuits for establishing andmaintaining substantially the some constant potential at all of saidpoints, which potential, if permitted to fluctuate or differ between anyof said points when the value of said one element is shifted, wouldcause transients to be generated, and

switching means for selectively coupling said second circuit to saidfirst circuit at said points for controlling the generation by saidoscillator at either one of said first and second frequencies.

12. A frequency shift oscillator as claimed in claim 8 and wherein saidfirst circuit includes a resistive and capacitive element connected inseries,

said second circuit including a resistive element connected between saidterminals to cause said oscillator to operate at said second frequencyhigher than said first frequency when said second circuit is selectivelycoupled to said first circuit.

13. A frequency shift oscillator as claimed in claim 8 and wherein saidcoupling means includes a switching means connected in series with saidsecond circuit between said points in said first circuit, and

means for placing said switching means in one state to couple saidsecond circuit and in a second state to remove said second circuit frombetween said points.

14. A frequency shift oscillator as claimed in claim 9 and wherein saidsecond circuit includes a first switching device in series with a thirdresistive element, the series arrangement of said first device and saidthird resistive element being connected across said first resistiveelement, and further including a second switching device in series witha fourth resistive element, the series arrangement of said second deviceand said fourth resistive element being connected across said secondresistive element,

said placing means serving to selectively operate said switching devicesto complete the connection of said third resistive element through saidfirst device across 9 10 said first resistive element and the connectionof sz id FOREIGN PATENTS f gzgg j 'g fg xfii' f 1,151,011 7/1963 Germany331-119 0 1,093,538 12/1967 Great Britain 331179 References Cited1,109,450 4/1968 Great Britain 331-179 5 The following references, citedby the Examiner, are of record in the patented file of this patent orthe original ALFRED BRODY' Pr'mary Examiner patent UNITED STATES PATENTS2,617,035 11/1952 Janssen at a] 331-419 3,514,111 5/1910 Rose =1 a1. 331119 x 3,363,204 1/1968 Kageyama et a]. 325-163 X

